This invention pertains to oscillators and, more particularly, to a voltage controlled ring oscillator and the control circuitry for programming the oscillator.
In a ring oscillator, the number of delay elements connected in a closed loop, as well as the propagation delay of each of those elements, determines the frequency of oscillation; or, if the ring oscillator is voltage controlled, the center frequency of oscillation. Consequently, for a given number of delay elements connected in a closed loop, higher frequency of operation may be achieved if the propagation delay through each element can be reduced.
If the number of delay elements in the closed loop can be changed, the frequency of oscillation can be made to be programmable. But, switching in or out delay elements can only change the frequency of the oscillator in incremental steps. Therefore, if the propagation delay through each delay element can be reduced, better resolution, i.e., finer adjustments in the frequency of the oscillator can also be achieved.
Accordingly, the invention described below achieves both high frequency of operation and frequency programmability in small incremental steps by the use of a delay element that consists of two, two-input NOR gates. Furthermore, the NOR gates are implemented in a bipolar structure which only requires the input signal to the gate to change the state of one transistor in order to effect a change in the output of the gate. Consequently, the propagation delay through each of the gates is minimal and the number of transistors needed to implement the oscillator circuit is also minimal.